less

Less is a dynamic stylesheet language that extends CSS with features intended to improve maintainability and reuse. It adds variables, mixins, nesting, and operations, allowing developers to write more modular and concise code that compiles to standard CSS. Files use the .less extension and are compiled to CSS either on the server with a command-line tool or in the browser via a JavaScript library.

Key features include variables (prefixed with @) for reusable values such as colors and sizes, mixins for reusable groups of declarations (which can take parameters), and nested rules that mirror the structure of the HTML. Less also supports operations for arithmetic and color manipulation, as well as imports to compose styles from multiple files. In addition, mixins can use guard conditions to perform simple logic during the compilation process. Since any valid CSS is valid Less, existing CSS can be incrementally adopted.

History and usage: Less was created by Alexis Sellier in 2009 as an open-source project to address CSS’s limitations. It gained early traction in front-end workflows and remains a widely used option among CSS preprocessors, alongside Sass and Stylus. The ecosystem includes less.js for client-side compilation and various build-tool integrations with systems like Grunt, Gulp, and Webpack. While modern CSS features such as custom properties reduce the need for preprocessing in some contexts, Less remains in use where its features align with project workflows.

See also: CSS preprocessor; Sass; Stylus.